A mobile telephone terminal is more convenient than a fixed-line phone in that a user can freely carry the mobile telephone terminal. However, the mobile telephone terminal can easily be lost or stolen. For that reason, in the past, there was suggested a technique of a mobile telephone terminal having a lock function of forcibly locking operation of the mobile telephone terminal in a remote manner by e-mail in order to prevent a third party from illegitimately using the mobile telephone terminal which the user had lost or which had been stolen (see, for example, Patent Document 1).
In the technique disclosed in Patent Document 1, a lock code and a private code are registered in an e-mail to be sent, and a mobile telephone terminal receiving the e-mail analyzes the received e-mail. When the lock code or the private code accords with information set in advance, the operation of the mobile telephone terminal is configured to be forcibly locked.    Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2003-224886